Furan resin binders, that is resins produced from furfuryl alcohol, have found world-wide use as sand binders in the production of foundry sand shapes. These furan resins typically undergo rapid polymerization or curing in the presence of acid catalyst.
One of the problems encountered in the foundry in connection with the use of furan resin has been the fact that the resins are indeed so reactive in acid systems. The premixing of catalyst, and binder with the sand prior to placement of the sand mix in sand shaping elements sometimes results in a mixture with relatively short "bench life." A recent development which appears to be gaining world-wide acceptance is described in U.S. Pat. No. 3,879,339 issued to Richard, involves the elimination of the acid catalyst from the formulation of the sand-binder system which exists prior to placement in the sand shaping element. In the latter method the binder and sand are admixed to include a peroxide to provide a sand, binder, and peroxide mixture which contains no polymerization catalyst. This mixture is placed in the shaping element. Thereafter, the thus shaped sand mix is exposed to sulfur dioxide gas or other oxidizable gaseous catalyst, for example, by blowing a gas mixture containing sulfur dioxide or other oxidizable gaseous catalyst through the sand shape. Apparently because of the presence of the peroxide, an acid catalyst is formed instantaneously from the sulfur dioxide and the sand shape substantially instantaneously is cured into a high strength integral solid.
However, even though the peroxide-binder-sand mixes which are used in accordance with the Richard method contain no catalyst it has been found that these mixes may not exhibit prolonged bench life when hydrogen peroxide, for example, is used as the peroxide. For example, a freshly mixed hydrogen peroxide-binder-sand mixture when gassed with SO.sub.2 will typically produce sand shapes having tensile strength in order of 100-300 psi. On the other hand, these same uncured sand mixes, when placed, for example, 3-6 hours after they are mixed, and when then gassed with the same level of SO.sub.2, often produced sand shapes which are either totally uncured, or are of inadequate tensile strength to use in a foundry within a reasonable period of cure time. The whole point of the SO.sub.2 gassing method is to produce a sand shape which is instantaneously useable, and a major advantage of the SO.sub.2 gassing method is lost unless, as an immediate consequence of the gassing step, the sand shape is of sufficient strength to be handleable and to be useful in the foundry.
On the other hand, many, if not most, foundries are set up to involve day-to-day shift operations in which the sand mixes are produced in very large quantities at one or two times during the work shift. The pre-prepared mixes are stored in large hoppers and are dispensed, from time to time, for distribution to a work station at which portions of the sand pre-mix are placed in sand shaping elements and gassed.
Thus, it has been unfortunate that, when the peroxide used is the most desirable peroxide from an economic viewpoint, i.e. hydrogen peroxide, it has been found that the pre-made peroxide-binder-sand mixes do not reliably retain their "vitality" for use in the SO.sub.2 gassing process for the periods of time which would be amenable to usual foundry practices, particularly when some lower quality sands are employed. It is unfortunate that, in such systems heretofore available, as the hours pass portions of such sand mixes taken from the main storage hopper were found to have produced sand shapes of decreasing tensile strength, until finally the sand shapes formed therefrom remain virtually unhandleable immediately after the gassing step.
It would be highly desirable to improve the technology surrounding the SO.sub.2 gassing method to provide sand mixes which will retain their "vitality" and remain useable throughout at least a half of a working shift, and preferably throughout an entire working shift, e.g. 4-8 hours.
On the other hand, the improvement in the technology must be very economical and entirely amenable and adaptable to routine foundry practice. In addition, whatever changes are involved should not result in a substantial lessening of the tensile strengths of the sand shapes produced in the method, but result in a prolongation of the period of time known as bench life, in which high strength sand shapes are produced immediately upon the SO.sub.2 gassing step.